Ordnance such as guided artillery projectiles, guided missiles, and guided bombs, all of which will be referred to herein as “projectiles,” may include a variety of imaging or non-imaging sensors to detect and track potential targets. Sensors used to guide projectiles to an intended target are commonly referred to as seekers. Seekers may operate in various portions of the electromagnetic spectrum, including the visible, infrared (IR), microwave, and millimeter wave (MMW) portions of the spectrum. Some projectiles may incorporate multiple sensors that operate in more than one portion of the spectrum. A seeker that incorporates multiple sensors that share a common aperture and/or common optical system is commonly called a multimode seeker.
One type of seeker used in projectiles is a semi-active laser (SAL) seeker to detect laser radiation reflected from an intended target and to provide signals indicative of the target bearing such that the projectile can be guided to the target. The SAL may include an optical system to capture and focus the reflected laser radiation and a detector. In order to provide high sensitivity, the SAL optical system may have a large aperture and high optical efficiency. In order to guide a projectile to a target when laser illumination of the target is not available, the projectile may be equipped with a dual-mode seeker including a SAL seeker and an imaging infrared (IIR) seeker.
Conventional shared aperture dual-mode or tri-mode seekers use Cassegrain-like catadioptic systems. These systems have a relatively narrow field of view, for example approximately ±3°, and are therefore typically gimbaled so as to allow a wider field of regard to be obtained by moving the system. As will be appreciated by those skilled in the art, the term “field of view” refers to the angular coverage that the optical system design supports, while the term “field of regard” refers to the angular coverage that is achievable by gimbaling. For example, the above-mentioned Cassegrain-type systems typically have a field of view of a few degrees, but may have a field of regard of ±30 degrees, with the field of regard limits generally determined by the gimbal design. These systems also have large obscurations which waste a significant portion (for example, approximately 25%) of the collection aperture area.